Polyisocyanate pre-polymer resins have proven to be useful in a wide variety of applications, such as paper impregnation, concrete formwork coatings, wood composite adhesives, foundry binders, and polyurethane foams. The applications take advantage of many of the useful properties of isocyanates such as reactivity, strength, flexibility, water resistance, flame resistance, and being free of formaldehyde.
As examples of such applications, there have been uses of an uncatalyzed polyfunctional isocyanate, preferably MDI, to impregnate a cellulosic substrate that is pressed at high temperature and pressure, useful for forming overlays and strong, water resistant laminates. In a particular application, U.S. Pat. No. 5,008,359 describes an implementation in which cure occurs in less than 5 minutes at temperatures between 105 and 245° C. and in between 1 and 2 minutes at temperatures over 150° C. U.S. Pat. No. 5,580,922 discloses that impregnation of paper with MDI diluted with triacetin and/or triethyl citrate will cure at ambient conditions in times as short as 1-2 days to an insoluble polyurea. The impregnated paper has relatively low residual amounts of undesirable isocyanates and 4,4′-diaminophenylmethane present.
In another particular application, U.S. Patent Application No. 20060182979A1 discloses a method of forming and curing a polyisocyanate impregnated fibrous sheet. The sheet is first impregnated with an isocyanate polymerization catalyst in alcohol solution, then dried, then treated with a polyisocyanate composition, and cured. The alcohol solvent drying step occurs at temperatures between 200 and 300° F. The curing step requires heating the polyisocyanate impregnated fibrous sheet to a temperature between 250 and 350° F. The use of an alcohol solvent that must be removed to place a catalyst in the fibrous sheet adds undesirable cost to the product.